Adjustable rotary jar



Sept. 3, 1935. G. w. BOWEN ADJUSTABLE ROTARY JAR 2 Sheets-Shet 1 FiledSept. 15, 1934 p 3, 1935. G. w. BOWEN 2,013,127

ADJUSTABLE ROTARY JAR Filed Sept. 15, 1954 2 Sheets-Sheet 2 6% .5 Q93 9.cg 1a Patented Sept. 3, 1935 UNITED STATES PATENT oFFicE ADJUSTABLEROTARY JAR Application September 15, 1934, Serial No. 744,167

11 Claims.

My invention relates to jars for use in drilling and operating oil wellsand the like, and has particular reference to a jarring tool which isreadily adjustable to exert jarring blows of predetermined intensity.

In the oil 'well industry the various earth boring tools employed indrilling an oil well frequently" become stuck in the hole and it isnecessary to jar or otherwise produce one or more shocks in the drillingstring to dislodge the tool which is stuck, permitting such tool to beeither Withdrawn from the hole or to permit the tool to proceed with itsnormal drilling functions. A

number of different jarring devices have been developed for thispurpose, the most successful being a jar which is employed in rotarydrilling consisting essentially of a bowl or shell attached to adrilling string, with a mandrel located within the bowl and attached tothe drilling tools with suitable separable connecting devices normallyholding the mandrel against longitudinal movement with respect to thebowl until a predetermined longitudinal strain is exerted between thedrill string and a tool which may be stuck in the hole.

Some of such prior devices have been provided with adjustmentsQrmechanism by which the amount of strain required. to release themandrel and bowl connecting means can be varied; but all of such devicesrequire that, the entire drill string and the jar be removed from thehole before the adjusting devices can be actuated to vary the requiredstrain.

It is', therefore, an object of my invention to provide a rotary jar inwhich the mandrel and bowl may be connected against longitudinalmovement by a separable connecting mechanism and in which suchconnecting mechanism can be adjusted to vary the required strain and tovary the tonnage of the jarring blow without the necessity of removingthe drill string or the jar from the well.

Another object of my invention is to provide a tool, as set forth in thepreceding paragraph, in which the connecting means may be adjusted tovary the tonnage of the blow from the top of the well.

Another object of my invention is to provide a jar, of the character setforth, in which the connecting mechanism between the mandrel and bowlcomprises a spring sleeve surrounding a portion of the mandrel and inwhich an adjusting device is employed to radially move the sleeve towardand away from the mandrel to vary the gripping relation therebetween.

Another object of my invention is to provide a tool, of the characterset forth in the preceding paragraph, in which the adjustment mechanismmay be operated by rotation of the drill string without the necessity ofremoving the drill string and tool from the hole.

Other objects of my invention will be apparent from a study of thefollowing specification read in connection with the accompanyingdrawings, wherein:

Fig. 1 is a vertical sectional view of a jarring tool constructed inaccordance with my invention, showing the tool in contracted or drillingposition.

Fig. 2 is a vertical sectional view similar to Figure 1, showing thetool in jarring position.

Fig. 3 is an enlarged sectional view similar to Figures 1 and 2, showingthe tool in adjusting position.

Fig; 4 is a detail elevational view, partly in section, of the springsleeve employed as the mandrel and bowl connecting means in practicingmy invention.

Fig. 5 is a plan view of the shown in Figure 4.

Fig. 6 is a horizontal sectional view, taken along the line VI-VI ofFigure 3.

Fig. 7 is a. detail horizontal sectional View, taken along line VII-VIIof Figure 3.

Fig. 8 is a vertical sectional view of a modified form of jar,constructed in accordance with my invention, illustrating the jar in itsfully extended or jarring position.

Fig. 9 is a view similar to Figure 8, illustrating spring sleeve,

the jar in the adjusting position, while- Fig. 10 is a view similar toFigures 8 and 9, illustrating the jar in its fully contracted positionready for drilling.

Referring to the drawings, I have illustrated my jar as comprising apair of telescopically assembled members, one being a bowl or shell Ithreadedly connected, as at 2, to an upper sub 3, which is provided witha threaded box 4 to receive the threaded pin on the lower end of astring of drill pipe (not shown), such as is ordinarily employed inrotary drilling of oil wells and the like, and the other being a mandrel5.

The shell or bowl I is illustrated as being hollow to receive thereinthe mandrel 5, the lower end of which is formed as a threaded pin 6 tobe received in the threaded box of either additional sections of drillpipe or the upper end of drill bits, reamers, other drilling toolsemployed in drilling the well, or fishing tools of all types.

I flanges I8 on the friction head I1.

In order to permit rotary forces exerted upon the drill string to rotatethe drilling tools, I provide the lower end of the shell or bowl I .witha lower sub I, the extreme lower end of which is provided with aplurality of extending fingers .8 to be received in a plurality of slots9 formed upon an enlarged section III of the mandrel 5 so that whenthemandrel and bowl are .in .contracted position, as shown in Figure 1,the fingers 8 and slots 9 constitute a clutch. transmitting rotaryforces from the bowl I to the mandrel 9. But when the bowl I is liftedrelative to the mandrel, as shown in Figure 2, the fingers 8 willseparate from the slots 9 and permit the bowl to be rotated relative tothe mandrel.

The mandrel 5 is provided with an enlarged head II located within thebody of the bowl I having a shoulder I2 formed upon the lower endthereto to constitute a hammer to engage shoulders I3 formed upon thelower sub I, the shoulders I2 and I3 constituting hammer and anvilabutting members respectively to transmit jarring impacts from the bowlI to the mandrel 5 when the bowl I is rapidly lifted relative to themandrel 5.

From an inspection of Figures 1, 2 and 7, it will be observed that theexterior surface of the mandrel head II is provided with a plurality oflongitudinally extending slots I4 to engage a plurality oflongitudinally extending splines I5 formed upon and projecting inwardlyfrom the interior surface of the bowl I. As shown in Figure 1, wheneverthe mandrel and bowl are in contracted position, the splines I5 willengage the slots I4 and assist in the transmission of rotary forcesbetween the bowl and mandrel.

Likewise a similar series of splines I6 formed upon the upper end of thelower sub I extending above the anvil shoulders I3 will engage the slotsI4 when the mandrel and bowl are moved to extended or jarring position,as shown in Figure 2, so that when the bowl and the mandrel are ineither fully contracted position, as shown in Figure l, or in fullyextended position, as shown in Figure 2, the mandrel and bowl areconnected against relative rotarymovement so that in either of thesepositions rotary forces exerted upon the drill string will tend to causerotary movement of the drilling tools.

The extreme upper end of the mandrel 5 is illustrated as being providedwith a friction head I! having a plurality of radially extending flangesI8 formed thereon and extending around the friction head IT. Thefriction head I! is adapted to be received within a spring frictionsleeve I9 which has radially extending flanges 20 formed upon theinterior thereof to engage below the It will be understood by thoseskilled in the art that these flanges will extend radially but arelatively small distance, a matter of a few hundredths of an inch, sothat they will constitute gripping members positively holding the springfriction sleeve I9 against longitudinal movement on the friction headI'I until longitudinal forces are exerted between the sleeve I9 and thehead I! sufficient to force the walls of the spring sleeve outwardly asuflicient distance to allow the flanges I8 and 20 to pass each other.

It will also be observed that the exterior of the spring sleeve I9 isprovided with a plurality of radially extending flanges 2|, whichflanges, however, are disposed longitudinally along the spring sleeve I9in spaced relation relative to the flanges 29 on the sleeve I9 so thatthe section By referring particularly to Figure 4, it will be observedthat the spring sleeve I9, while made of suitable resilient material,such as hardened steel, and would, therefore, have considerableresiliency, the resiliency of this sleeve may be further increased byproviding a plurality of longitudinally extending slots 22 and 23therein. In

the form illustrated herein, slots 22 extend from the lower end of thesleeve I9 to a point spaced from the upper end of the sleeve, whileslots 23 extend from the upper end of the sleeve to a point spaced fromthe lower end thereof, thus dividing the sleeve into a plurality ofspring strips which, however, are joined at the upper end and the lowerend of the sleeve so that the entire sleeve remains as a sleeve unit.

By referring particularly to Figures 1, 2 and 3, it will be observedthat the outer flanges 2I on the sleeve I9 bear against the taperedinterior surface of an adjusting sleeve 24, the exterior surface ofwhich is substantially parallel to the exterior cylindrical surface ofthe bowl I. The lower end of the adjusting sleeve 24 is provided withthreads 25 engaged with interior threads 26 formed upon the interior ofthe bowl I so that by rotating the bowl I relative to the sleeve '24,the sleeve will be moved longitudinally in the bowl upon its threads 25and 26 and thus, by reason of the tapered interior surface of theadjusting sleeve 24, will exert a radial pressure upon the spring sleeveIS, the value of which will depend upon the longitudinal position of thesleeve relative to the bowl.

It will thus be apparent to those skilled in the art that with themandrel in the contracted position shown in Figure 1 and with theadjusting sleeve 24 in a predetermined position longitudinally of thebowl I, it will require a predetermined longitudinal strain between thebcwl I and the mandrel 5 to pull the flanges I8 of the friction head I'Ipast the flanges 29 of the spring sleeve I9. But, if the adjustingsleeve 24 is raised relative to the bowl I, it will require a greaterstrain to pull the flanges I8 past the flanges 20.

By referring particularly to Figure 3, it will be observed that thelower end of the adjusting sleeve is provided with an inwardly extendingspline 21 engageable in an elongated key-way 28 cut into and extendinglongitudinally of the friction head I1 so that in all positions of themandrel relative to the bowl I the mandrel is positively keyed to theadjusting sleeve 24.

It will also be observed that the splines I5 are in spaced relation tothe beginnings of the splines I6, this space being sufflcient toaccommodate the entire length of the hammer head II of the mandrel 5 sothat when the mandrel is in the position shown in Figure 3, that ishalf-way between the contracted position shown in Figure 1 and the fullyextended or jarring position shown in Figure 2, the head I I of themandrel is free to rotate relative to the bowl and thus, with themandrel and bowl in this position, rotary movement of the drill stringwill cause rotary movement of the bowl I relative to the adjustingsleeve 24 without causing rotary movement of the lower sub I0 and itsattached drilling tools. Thus the adjusting sleeve 24 may be initiallyassembled in the bowl I to exert a minimum desired pressure upon thefriction sleeve l9 so that a minimum amount of longitudinal strain onthe drill string will be sufilcient to separate the spring sleeve 19 andthe friction head I! and thus jar the hammer head ll against the anvill3. If such minimum amount of strain and minimumimpact is not sufficientto dislodge a stuck tool,

the drill string may be lowered slightly relative to the mandrel whilethe drill string is given a slight rotary torque. This will cause theadjusting sleeve 24 to be screwed upwardly in the bowl I as soon as theslots Mof the mandrel head II disergage splines I6 and before engagingsplines l5. By regulating the amount of torque placed in the drillstring, a selected upward movement of the adjusting sleeve 24 may beaccomplished to increase the strain required to again jar the tool toany predetermined increased value. This entire adjusting operation canbe accomplished without disconnecting the drill string. withoutwithdrawing the drill string from the hole, and without withdrawing thejar from the hole.

This advantage will be appreciated by those skilled in the art when itis considered that during the drilling operations the most frequentopportunities for sticking or locking of the tools in the hole occurswhen the hole has penetrated to a considerable distance below thesurface of the ground and that it is a long and arduous operation towithdraw the long drill string and to withdraw the jar from the hole inorder to make adjustments for increased tonnage of blow'when the initialsetting of the jar is found insufiicient to dislodge the tool. Moreover,in order to withdraw the jar and drill string from the hole, it isnecessary to disconnect the jar from the drilling tools and after theadjustment is made, the drill string is again lowered into the hole inthe hope that it will reengage the stuck drilling tools and reconnectwith these tools at a distance which may be several thousand feet fromthe surface of the ground.

After the jar has been moved to extended or jarred position, as shown inFigure 2, and it is desired to again contract the tool ready for a newjarring operation, it is desirable that relatively little downwardpressure on the bowl I! should reconnect the spring sleeve is and thefriction head H. To accomplish this relatively easy reconnection, I haveprovided a radially extending shoulder 29 on the upper end of the springsleeve l9 to engage a fixed ring 30 secured to the bowl I, the springsleeve 19 being undercut for a distance longitudinally thereof suficientto permit the sleeve to raise slightly in the bowl and thus separate thespring sleeve from the adjusting sleeve 25. With the spring sleeve soelevated, comparatively little force need be exerted to move the flangesB8 of the friction head l'l past the flanges Ell of the spring sleeve l9and, as soon as the flanges have passed each other, a spring 3!! securedabove the spring sleeve l9 forces the spring sleeve down in the bowluntil the shoulder 29 engages the ring 30 and the spring sleeve is againin the contracted or locked position ready for a second jarringoperation.

Referring particularly to Figs. 8, 9 and 10, I have illustrated amodified form of the jarring tool, which comprises essentially an uppersub 3a, the lower end of which is connected to an outer bowl or shellla, at the extreme lower end of which is secured a lower sub la, theparts Ia, 3a, and la being identical in construction-with the parts I, 3and 1, illustrated in the form of the device shown in Figs. 1 to 7.

Assembled within the shell or bowl la for telestriking head ll, shown inFigs. 1 to 7, the-head 4 I la having formed thereon suitable shouldersHa constituting hammers which will engage similar shoulders [3a formedupon the lower sub 10,, so that when the mandrel and shell are in theextended positions, as shown in Fig. 8, the shoulders l 2a and IM willbe brought into engagement with each other to produce the jarring blow.

Like the mandrel head it, the head lid is formed with a plurality oflongitudinally extending slots Ma adapted to engage a plurality oflongitudinally extending splines lea formed upon and projecting inwardlyof the interior of the shell Ia when the mandrel and shell are incontracted position, as shown in Fig. 10. Similarly,

the lower sub la is provided with longitudinally extending splines ltdto engage the slots Ma, of the mandrel head when the hole and mandrelare in fully extended position, as shown in Fig. 8.

Secured to the mandrel immediately above the head i la is a frictionhead Ila having on its exterior surface a plurality of friction flangesma adapted to engage similar friction flanges 20a. formed upon theinterior of a spring friction sleeve i911 assembled within the shell orbowl la.

In this form of the invention the friction sleeve 09 is illustrated asresting directly upon a collar Ma comprising a simple cylindricalcollar, the exterior of which is provided with threads 26a en gagingsuitable threads formed upon the interior of the bowl or shell la. Thecollar 241a, like the adjusting sleeve 26, shown in Figs.- 1, 2 and 6,is provided with one or more radially extending splines 2M engageable insuitable longitudinal slots 28a formed in the exterior surface of thefriction head Ila.

In this form of the invention, theinterior wall of the shell or bowl lais provided with a tapering surface til formed directly thereon andconforming with the tapering exterior surface of the friction sleevelea. Thus, by moving the collar E la upwardly and downwardly relative tothe shell or bowl la, the spring sleeve 59a. may be lifted or loweredrelative to the tapering surface til to permit greater or lesser radialmovement of the portions of the sleeve Ella, and thus decreasing orincreasing, respectively, the amount of longitudinal strain which mustbe exerted between the shell la and the mandrel 5a in order to disengagethe flanges of the friction head We from the flanges of the springfriction sleeve iQa.

Similarly to the construction illustrated in Figs. 1 to 7, the midportion of the splines l5alta, are cut away, as indicated at M, so thatwhen the mandrel and shell are in a partially contracted position, asshown in Fig. 9, the enlarged head lla of the mandrel will disengage thesplines Ma and Ilia, permitting the shell la and the mandrel 5a to berotated relative to each other, while at the same time the engagement ofthe splines 2m with the slots of the friction head will permit suchrelative rotation to adjust the level of the collar 24a.

The operation of the form of device, illustrated in Figs. 8, 9 and 10,is similar to that shown in Figs. 1 to 7 in that with the jar in thefully contracted position, shown in Fig. 10, rotary movement transmittedthrough the drill pipe will be transmitted as rotary movement directfrom the shell la to the mandrel 5a. to produce the desired rotarymovement for the drilling operations. If the drill or other tool shouldbecome fast in the hole, longitudinal strain placed upon the drillstring will tend to pull the shell la upwardly relative to the mandrel5a against the resistance exerted between the friction sleeve Na and thefriction head l'la. Then as soon as sufficient strain has been taken tocause the flanges 20a and I8a to pass each other, the bowl la will moveupwardly with great rapidity, bringing the hammer and anvilshoulders |2aand I3a. into violent impact, producing the jarring blow necessary torelease the stuck tool.

If, as a result of the first jarring blow s produced, the tool does notbecome loosened, the drill string may be lowered until the enlarged headHa arrives at that point at which the splines l5a and Ifia are cut out,and rotary movement may be given the drill string and shell la in suchdirection as to move the collar 24a downwardly relative to the shell.Then the entire drill string may be lowered to move the shell and themandrel into fully contracted position, as shown in Fig. 10, reengagingthe flanges 20a and [8a, and a second jarring blow is produced which, inthis instance, will be a greater blow due to the fact that the frictionsleeve Illa has been permitted to move downwardly against the taperedwall 40 to a position in which the sleeve l9a is permitted less radialmovement than at the time of the first blow. Further tightening f thesleeve lOa may be accomplished in a similar manner if it is foundnecessary or desirable.

While I have shown and described the preferred embodiment of myinvention, I do not desire to be limited to any of the details set forthherein except as defined in the appended claims.

What I claim is:

l. A jar for use in rotary drilling comprising two memberstelescopically assembled one within the other for relative longitudinalmovement from a contracted position to an extended position, strikingabutments formed on said members engageable when said members are inextended position, disengageable means for coupling said memberstogether against rotation relative to each other, friction meansnormally holding said members in contracted position but releasable bythe exertion of a predetermined longitudinal strain between saidmembers, means for adjusting, said friction means to vary the strainrequired to release it, and means operable by r0- ,tation of one of saidmembers relative to the other of said members when said coupling meansis disengaged for changing the adjustment of said adjusting means.

2. A jar for use in rotary drilling comprising a bowl and a mandreltelescopically assembled one within the other for relative longitudinalmovement from a contracted position to an extended position, strikingabutments formed on said members engageable when said members are inextended position, disengageable means for coupling said memberstogether against rotation relative to each other, friction means on saidmandrel, friction means secured within said bowl for engaging saidmandrel friction means to normally hold said mandrel and bowl incontracted position but releasable by the exertion of a predeterminedlongitudinal strain between said bowl and said mandrel, means foradjusting the frictional relation between said mandrel friction meansand said bowl friction means, and means operable by rotation of one ofsaid members relative to the other of said members when said couplingmeans is disengaged for changing said adjusting means to varythe strainrequired to release it.

3. A jar for use in rotary drilling comprising a bowl and a mandreltelescopically assembled one within the other for movement from acontracted position to an extended position, cooperating strikingabutments formed on said bowl and upon said mandrel respectivelyengageable when said members are in extended position, disengageablemeans for coupling said bowl and said mandrel together against rotationrelative to each other, a sleeve secured in said bowl and surroundingsaid mandrel for frictionally engaging said mandrel to hold said mandreland. bowl in contracted position, means for exerting a radial pressureupon said sleeve, means mounting said pressure adjusting means formovement in said bowl to vary said radial pressure, and means operableresponsive to relative rotation between said bowl and said mandrel whensaid coupling means is disengaged for varying the position of saidadjusting means to vary the strain required to release said sleeve.

4. In a jar for use in rotary drilling comprising a bowl and a mandreltelescopically assembled one within the other for movement from acontracted position to an extended position, a friction sleeve securedin said bowl and surrounding said mandrel to frictionally hold saidmandrel and said bowl in contracted position, a tapered adjusting sleevesurrounding said friction sleeve and movable longitudinally relative tosaid friction sleeve to vary the strain required to release said sleevefrom said mandrel, means conmeeting said mandrel and said adjustmentsleeve to move said adjusting sleeve relative to said fric-.

tion sleeve upon rotation of said mandrel relative to said bowl wherebythe strain required to release said friction sleeve may be varied from apoint remote from said jar.

5. In a jar for use in rotary drilling comprising a bowl and a mandreltelescopically assembled one within the other for movement from acontracted position to an extended position, a friction sleeve securedin said bowl and surrounding said mandrel to frictionally hold saidmandrel and said bowl in contracted position, a tapered adjusting sleevesurrounding said friction sleeve and movable longitudinally relative tosaid friction sleeve to vary the strain required to release said sleevefrom said mandrel, means connecting said mandrel and said adjustmentsleeve to move said adjusting sleeve relative to said friction sleeveupon rotation of said mandrel relative to said bowl whereby the strainrequired to release said friction sleeve may be varied from a pointremote from said jar, interengaging means on said mandrel and said bowlfor preventing relative rotation of said mandrel and bowl when saidmandrel and bowl are in contracted position.

6. In a jar for use in rotary drilling compris,

ing a bowl and a mandrel telescopically assembled one within the otherfor movement from a contracted position to an extended position, afriction sleeve secured in said bowl and surrounding said mandrel tofrictionally hold said mandrel and said bowl in contracted position, atapered adjusting sleeve surrounding said friction sleeve and movablelongitudinally relative to said friction sleeve to vary the strainrequired to release said sleeve from said mandrel, means connecting saidmandrel and said adjustment sleeve to move said adjusting sleeverelative to said friction sleeve upon rotation of said mandrel relativeto said bowl whereby the strain required to release said friction sleevemay be varied from a point remote from said jar, splines on said bowlengageable with slots on said mandrel when said bowl and mandrel are ineither extended position or contracted position, said splines being freefrom said mandrel when said mandrel and bowl are in intermediatepositions.

7. A jar for use in rotary drilling, co'mpris of said members having ashoulder thereon movable radially withv respect to thefother of said.members, and a shoulder formed upon the other of said members engageablewith saidfirstn-amed shoulder, means variably limiting the. radialmovement of said first-named shoulder, and means operably responsive torotation of one of said members relative to the other of said memberswhen said coupling means is disengaged for .means for coupling saidmembers together against rotation relative to each other, a frictionsleeve in said bowl surrounding said mandrel to frictionally hold saidmandrel andsaid bowl in contracted position, a tapered surface formed insaid bowl surrounding and engaging said friction sleeve, means forlimiting the movement of said sleeve longitudinally of said bowl, meansmounting said limiting means for movement longitudinally of said bowl byrelative rotation between said mandrel and said bowl, and

means operable from a point remote from said jar for disengaging saidcoupling means and for ,rotating said mandrel and bowl relative to eachother.

9. In a jar ofor use in rotary drilling, comprising a bowl and'a mandreltelescopically assem bled one within the other-for relativelongitudinal.

movement from'a contracted'position to an extended position, strikingabutments formed upon said bowl and said mandrel engageable when saidmembers are in extended position, disengageable means for coupling saidmembers together against rotation relative to each other, a frictionsleeve in said bowl surrounding said mandrel to frictionally hold saidmandrel and said bowl in contracted position, a tapered surface formedin said bowl surrounding and engaging said friction sleeve, means forlimiting the movement of said sleeve longitudinally of said bowl, meansmounting said limiting means for movement longitudinally of said bowl byrelative rotation between said limiting means and said bowl, meansconnecting said limiting -means to said mandrel for rotation therewith,whereby relative rotary movement between said mandrel and said bowl whensaid coupling means is disengaged will move said limiting means in saidbowl.

, 10. In a jar for use in rotary drilling, comprising a bowl and amandrel telescopically assembled one within the other for relativelongitudinal movement from a contracted position to an extendedposition; striking abutments formed upon said bowland said mandrelengageable when said members 'arejin extended position, a frictionsleeve in said bowlfsurrounding said mandrel to frictionally hold sai'd'mandrel and said bowl in contracted position, a, tapered surface formedin said bowl surrounding and engaging said friction sleeve, means forlimiting the movement of said sleeve longitudinally of said bowl, meansmounting said limiting means for movement longitudinally of said 'bowlbyrelative rotation between said limiting means and said bowl, meansconnecting said limiting means to said mandrel for rotation therewith,whereby relative rotary movement between said mandrel and said bowl willmove said limiting means in said bowl, and interengaging means on saidbowl and said mandrel for preventing relative rotation of said mandreland said bowl when said mandrel and said bowl are in contracted positionand disengageable to permit said'relative rotation when said mandreland'said bowi are moved from contracted position.

. 11. -Ajjar for use in rotary drilling, comprising two memberstelescopically assembled one within the other for relative longitudinalmovement from bers to drilling tools, striking abutments formed on saidmembers engageable when said members are in extended position,interengaging means on said members for coupling said members togetherwhen in contracted position to transmit rotary force from one of saidmembers to the other thereof and disengageable when said members aremoved from contract-ed position to permit relative rotation between saidmembers, friction means normally holding said members in contractedposition but releasable by the exertion of a predetermined longitudinalstrain between said members, and means operably responsive to rela tiverotation between said members for adjuning said friction means to varythe strain 11;- quired to release it.

GEORGE WALTER BOWEN.

